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Ring Contraction of 1,2-Digermacyclohexa-1,4-diene to (Germacyclopent-3-enyl)germylene Norihisa Fukaya, Masaaki Ichinohe, Yoshio Kabe, and Akira Sekiguchi* Department of Chemistry, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan Received May 7, 2001 Summary: Thermolysis of cis,cis-1,6,7-trigerma-1,6,7tris(tri-tert-butylsilyl)-7-mesityl-3,4-dimethylbicyclo[4.1.0]hept-3-ene (1a) and cis,cis-1,6,7-trigerma-1,6,7-tris(tritert-butylsilyl)-7-mesityl-3-methylbicyclo[4.1.0]hept-3ene (1b) at 70 °C in benzene in the presence of diphenylacetylene and triethylsilane yielded products arising from (germacyclopent-3-enyl)germylene, which is formed by the ring contraction of 1,2-digermacyclohexa1,4-diene. Theoretical calculations on this contraction reaction leading from a six-membered ring with a GedGe double bond to a five-membered ring have also been performed. The chemistry of unsaturated cyclic compounds containing group 14 elements heavier than carbon is a recent subject of considerable interest.1,2 Synthetic methodologies and studies of the structural features of such compounds have developed quickly during the past 5 years.2 Ring contraction and expansion reactions of unsaturated cyclic compounds are very important and well-established in organic chemistry; however, such reactions are very rare in the chemistry of heavier group 14 elements.3 Recently, we have succeeded in synthesizing bicyclic 1,6,7-trigermabicyclo[4.1.0]hept-3-ene and 1,4,5-trigermabicyclo[2.1.0]pent-2-ene derivatives by a cycloaddition reaction of 2,3-dimethyl-1,3-butadiene or isoprene and phenylacetylene to the endocyclic GedGe double bond of mesityl-substituted cyclotrigermene.4 We presumed that the present fused bicyclic cyclotriger(1) (a) Tsumuraya, T.; Batcheller, S. A.; Masamune, S. Angew. Chem., Int. Ed. Engl. 1991, 30, 902. (b) Mackay, K. M. The Chemistry of Organic Germanium, Tin and Lead Compounds; Patai, S., Ed.; Wiley: New York, 1995; Chapter 2. (c) Driess, M.; Gru¨tzmacher, H. Angew. Chem., Int. Ed. Engl. 1996, 35, 828. (d) Baines, K. M.; Stibbs, W. G. Adv. Organomet. Chem. 1996, 39, 275. (e) Weidenbruch, M. Eur. J. Inorg. Chem. 1999, 373. (f) Power, P. P. Chem. Rev. 1999, 99, 3463. (g) Escudie´, J.; Ranaivonjatovo, H. Adv. Organomet. Chem. 1999, 44, 113. (2) (a) Cyclotrigermenes: Sekiguchi, A.; Yamazaki, H.; Kabuto, C.; Sakurai, H.; Nagase, S. J. Am. Chem. Soc. 1995, 117, 8025. (b) Sekiguchi, A.; Fukaya, N.; Ichinohe, M.; Takagi, N.; Nagase, S. J. Am. Chem. Soc. 1999, 121, 11587. Cyclotetrasilenes: (c) Kira, M.; Iwamoto, T.; Kabuto, C. J. Am. Chem. Soc. 1996, 118, 10303. (d) Wiberg, N.; Auner, H.; No¨th, H.; Knizek, J.; Polborn, K. Angew. Chem., Int. Ed. 1998, 36, 2869. Cyclotrisilenes: (e) Iwamoto, T.; Kabuto, C.; Kira, M. J. Am. Chem. Soc. 1999, 121, 886. (f) Ichinohe, M.; Matsuno, T.; Sekiguchi, A. Angew. Chem., Int. Ed. 1999, 38, 2194. Disilagermirenes: (g) Lee, V. Ya.; Ichinohe, M.; Sekiguchi, A.; Takagi, N.; Nagase, J. Am. Chem. Soc. 2000, 122, 9034. Spiropentasiladiene: (h) Iwamoto, T.; Tamura, M.; Kabuto. C.; Kira, M. Science 2000, 290, 504. Chalcogenatetrasilacyclopentenes: (i) Grybat, A.; Boomgaarden. S.; Saak, W.; Marsmann, H.; Weidenbruch, M. Angew. Chem., Int. Ed. 1999, 38, 2010. Cyclostannene: (j) Wiberg, N.; Lerner, H.-W.; Vasisht, S.-K.; Wagner, S.; Karaghiosoff, K.; No¨th, H.; Ponikwar, W. Eur. J. Inorg. Chem. 1999, 1211. Disilagermacyclopentadiene: (k) Lee, V. Ya.; Ichinohe, M.; Sekiguchi, A. J. Am. Chem. Soc. 2000, 122, 12604. (3) Isomerization of a 4-silamethylenecyclopropene derivative to a silacyclobutadiene derivative was reported; see: (a) Sakamoto, K.; Ogasawara, J.; Sakurai, H.; Kira, M. J. Am. Chem. Soc. 1997, 119, 3405. (b) Veszpre´mi, T.; Takahashi, M.; Ogasawara, J.; Sakamoto, K.; Kira, M. J. Am. Chem. Soc. 1998, 120, 2408.
Scheme 1
manes could be suitable precursors of the unsaturated germanium compounds by the cycloelimination reaction, since the thermal and photochemical generation of digermenes and germylenes from the cyclotrigermanes is well-established.1 Thus, we examined the thermal reaction of the 1,6,7-trigermabicyclo[4.1.0]hept-3-ene derivative in the presence of diphenylacetylene and triethylsilane and found an unprecedented ring contraction of a 1,2-digermacyclohexa-1,4-diene to a (germacyclopent-3-enyl)germylene derivative. We here report the first example of a facile ring contraction of unsaturated cyclic germanium compounds, studied from both the experimental and theoretical points of view. Thermolysis of a benzene solution of 1,6,7-trigermabicyclo[4.1.0]hept-3-ene (1a)4 for 6 h at 70 °C in a sealed tube in the presence of diphenylacetylene afforded colorless crystals of the germacyclopropene derivative 2 in 82% yield and pale yellow crystals of the germacyclopropenylgermacyclopentene derivative 3a in 53% yield (Scheme 1).5 Thermolysis of 1b with diphenylacetylene also gave 2 (88%) and 3b (58%). However, no evidence for the [2 + 2] reaction of the 1,2-digermacyclohexa-1,4-diene species 5 with diphenylacetylene to form the 1,6-digermabicyclo[4.2.0]octa-3,7-diene derivative was found. Interestingly, thermal reaction of 1a in the presence of phenylacetylene proceeded in a different way to give colorless crystals of 3,4-dimethyl-7-phenyl1,6-bis[tris(tri-tert-butylsilyl)]-1,6-digermabicyclo[4.2.0]octa-3,7-diene (7) in 77% yield, arising from [2 + 2] cycloaddition of 5a with phenylacetylene.6 (4) Fukaya, N.; Ichinohe, M.; Sekiguchi, A. Angew. Chem., Int. Ed. 2000, 39, 3881.
10.1021/om010369c CCC: $20.00 © 2001 American Chemical Society Publication on Web 07/13/2001
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Organometallics, Vol. 20, No. 16, 2001 3365 Scheme 2
The formation of products 2 and 3 clearly shows the existence of intermediate germylenes 4 and 6 during the thermolysis of 1 (Scheme 2). It seems that the 1,2digermacyclohexa-1,4-diene 5, which could be initially formed by the cycloelimination of 1, does not undergo [2 + 2] cycloaddition with diphenylacetylene, probably because of steric reasons. Alternatively, the resulting 5 undergoes a ring contraction to give the germylene 6, which then reacts with diphenylacetylene to form compound 3. The molecular structure of 3b was unambiguously characterized by X-ray crystallography, as shown in (5) Procedure of thermolysis of 1a in the presence of diphenylacetylene. The yellow crystals of 1a (50 mg, 0.049 mmol) and diphenylacetylene (40 mg, 0.22 mmol) were placed in a reaction tube with a magnetic stirrer. Dry degassed benzene (0.6 mL) was introduced by vacuum transfer, and then the tube was sealed. The mixture was heated for 6 h at 70 °C. The solvent was removed in vacuo, and the resulting residue was separated by gel permeation chromatography to give 2 (23 mg, 82%) and 3a (21 mg, 53%). 2: colorless crystals; mp 145-148 °C; 1H NMR (C6D6, δ) 1.25 (s, 27 H), 2.06 (s, 3 H), 2.85 (s, 6 H), 6.72 (s, 2 H), 7.02 (t, J ) 7 Hz, 2 H), 7.16 (t, J ) 7 Hz, 4 H), 7.68 (d, J ) 7 Hz, 4 H); 13C NMR (C D , δ) 21.0, 25.1, 26.5, 31.8, 127.1, 128.4, 128.5, 128.7, 6 6 136.4, 137.8, 142.5, 143.4, 148.9; 29Si NMR (C6D6, δ) 25.3. Anal. Calcd for C35H48GeSi: C, 73.82; H, 8.50. Found: C, 73.35; H, 8.48. 3a: pale yellow crystals; mp 194-197 °C; 1H NMR (C6D6, δ) 1.13 (s, 27 H), 1.25 (s, 27 H), 1.87 (s, 6 H), 2.34 (d, J ) 17 Hz 2 H), 2.40 (d, J ) 17 Hz, 2 H), 7.05 (t, J ) 7 Hz, 2 H), 7.24 (t, J ) 7 Hz, 4 H), 7.92 (d, J ) 7 Hz, 4H); 13C NMR (C6D6 , δ) 19.8, 24.7 (2C), 31.9, 32.1, 32.4, 127.6, 128.5, 130.2, 132.4, 135.3, 144.0; 29Si NMR (C6D6, δ) 27.7, 36.5. Anal. Calcd for C44H74Ge2Si2: C, 65.70; H, 9.27. Found: C, 65.32; H, 9.14. (6) Spectroscopic data for 7: mp 179-182 °C; 1H NMR (C6D6, δ) 1.19 (s, 27 H), 1.30 (s, 27 H), 1.88 (s, 3 H), 2.04 (s, 3 H), 2.11-2.18 (m, 2 H), 2.32-2.41 (m, 1 H), 2.69-2.74 (m, 1 H), 7.00 (t, J ) 9 Hz, 2 H), 7.12 (t, J ) 9 Hz, 1 H), 7.25 (s, 1 H), 7.32 (d, J ) 9 Hz, 2 H); 13C NMR (C6D6, δ) 22.1, 22.5, 24.5, 24.6, 30.6, 31.5, 31.8, 31.9, 123.1, 125.5, 126.4, 126.7, 128.1, 146.1, 151.4, 170.3; 29Si NMR (C6D6, δ) 27.9, 29.7. Anal. Calcd for C38H70Ge2Si2: C, 62.67; H, 9.69. Found: C, 63.16; H, 9.32. Crystal data for 7‚0.5(toluene) at 120 K: molecular formula C38H70Ge2Si2‚0.5C7H8, mol wt 774.37, triclinic, a ) 9.3000(8) Å, b ) 12.303(1) Å, c )19.992(1) Å, R ) 75.719(5)°, β ) 85.135(5)°, γ ) 74.544(4)°, V ) 2136.1(3) Å3, space group P1 h , Z ) 2, Dcalcd ) 1.204 g/cm3. The final R factor was 0.0499 (Rw ) 0.1301 for all data) for 9484 reflections with I > 2σ(I). GOF ) 1.042.
Figure 1.7 The five-membered ring is nearly planar (sum of interior angles 538.9°). Although the Ge-C bond lengths of the three-membered ring (1.980(3) and 1.985(3) Å) slightly exceed the upper limit of literature values of germacyclopropenes (typically 1.88-1.94 Å), the CdC double bond (1.328(5) Å) is relatively short (typically 1.33-1.39 Å).8 The Ge-Ge bond (2.4922(5) Å) and Ge-Si bonds (2.449(1) and 2.439(1) Å) are elongated by the steric repulsion. To confirm the formation of the germylene 6, we have also examined thermolysis of 1 in the presence of triethylsilane, which can react exclusively with the divalent species. Thus, thermolysis of 1a with an excess of triethylsilane for 6 h at 70 °C yielded (tri-tertbutylsilyl)(triethylsilyl)(mesityl)germane (8) in 86% yield and digermane derivative 9 in 83% yield (Scheme 3).9 These compounds appear to be derived from the insertion of the resulting germylene 4 and (germacyclopentenyl)germylene 6a into the Si-H bond of triethylsilane, respectively. Baines et al. reported the facile digermene-to-germylgermylene and germasilene-to-silylgermylene rearrangements by 1,2-mesityl shifts of tetramesityldigermene (7) Single crystals of 3b were obtained by recrystallization from hexane. Crystal data for 3b at 120 K: molecular formula C43H72Ge2Si2, mol wt 790.37, monoclinic, a ) 14.6250(8) Å, b ) 14.6190(7) Å, c ) 20.1150(11) Å, β ) 91.077(4)°, V ) 4299.9(4) Å3, space group P21/n, Z ) 4, Dcalcd ) 1.221 g/cm3. The final R factor was 0.0559 (Rw ) 0.1631 for all data) for 10 018 reflections with I > 2σ(I). GOF ) 1.091. The structure was solved by the direct method and refined by the fullmatrix least-squares method using the SHELXL-97 program. The position of the carbon atom of the methyl group attached to the CdC double bond of the five-membered ring is disordered between the C40 and C41 atoms, and the molecular structure with the largest occupancy (70%, C43 bonded to C40) is shown in Figure 1. (8) (a) Egorov, M. P.; Kolesnikov, S. P.; Struchkov, Y. T.; Antipin, M. Y.; Sereda, S. V.; Nefedov, O. M. J. Organomet. Chem. 1985, 290, C27. (b) Ando, W.; Ohgaki, H.; Kabe, Y. Angew. Chem., Int. Ed. Engl. 1994, 33, 659. (c) Tokitoh, N.; Kishikawa, K.; Matsumoto, T.; Okazaki, R. Chem. Lett. 1995, 827.
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Table 1. Relative Energies (in kcal/mol) of the Digermenes, the Corresponding Germylgermylene Isomers, and the Transition States at the B3LYP/6-31G* Level Calculations
Scheme 3
Figure 1. Molecular structure of 3b with thermal ellipsoids drawn at the 30% level (hydrogen atoms are omitted for clarity). Selected bond distances (Å): Ge1-Ge2 ) 2.4922(5), Ge1-Si1 ) 2.449(1), Ge2-Si2 ) 2.439(1), Ge1-C25 ) 1.980(3), Ge1-C26 ) 1.985(3), C25-C26 ) 1.328(5). Selected bond angles (deg): C25-Ge1-C26 ) 39.2(1), C26-C25-Ge1 ) 70.7(2), C25-C26-Ge1 ) 70.2(2), Si1-Ge1-Ge2 ) 134.0(0), Si2-Ge2-Ge1 ) 121.6(0), C39-Ge2-C42 ) 91.3(1).
and tetramesitylgermasilene, respectively.10,11 Since the present system is quite interesting from the viewpoint of the unusual contraction reaction leading from a sixmembered ring with a GedGe double bond to a fivemembered ring, we have performed theoretical calculations on this system.12 Barriers to isomerization of the three prototype digermenes, H2GedGeH2, Me(H)Ged GeH2, and the parent 1,2-digermacyclohexa-1,4-diene, to the corresponding germylgermylenes have been determined by DFT calculations at the B3LYP level with (9) Spectroscopic data for 8 and 9: 8: colorless oil; 1H NMR (C6D6, δ) 0.84-0.90 (m, 6 H), 0.95-0.98 (m, 9 H), 1.23 (s, 27 H), 2.10 (s, 3 H), 2.55 (s, 3 H), 2.65 (s, 3 H), 4.23 (s, 1 H), 6.78 (s, 1 H), 6.87 (s, 1 H); 13C NMR (C6D6, δ) 6.8, 8.5, 21.0, 24.8, 27.5 (2C), 31.8, 128.2, 129.7, 136.8, 137.9, 142.8, 143.9; 29Si NMR (C6D6, δ) 4.6, 28.1. Anal. Calcd for C27H54GeSi2: C, 63.90; H, 10.72. Found: C, 63.56; H, 10.44. 9: colorless solid; mp 96-99 °C; 1H NMR (C6D6, δ) 1.05-1.09 (m, 6 H), 1.13-1.18 (m, 9 H), 1.29 (s, 27 H), 1.30 (s, 27 H), 1.78 (s, 6 H), 2.49 (d, J ) 17 Hz, 2 H), 2.60 (d, J ) 17 Hz, 2 H), 3.29 (s, 1 H); 13C NMR (C6D6, δ) 8.2, 8.9, 19.4 (2C), 24.3, 26.0, 32.0, 32.4, 32.7, 36.9, 131.9, 132.3; 29Si NMR (C6D6, δ) 9.1, 25.5, 40.2. Anal. Calcd for C36H80Ge2Si3: C, 58.24; H, 10.86. Found: C, 58.04; H, 10.54. (10) (a) Baines, K. M.; Cooke, J. A. Organometallics 1992, 11, 3487. (b) Baines, K. M.; Cooke, J. A.; Vittal, J. J. J. Chem. Soc., Chem. Commun. 1992, 1484. (11) Baines, K. M.; Cooke, J. A.; Dixon, C. E.; Liu, H. W.; Netherton, M. R. Organometallics 1994, 13, 631. (12) A theoretical calculation on the isomerization of H2GedGeH2 to HG ¨ eGeH3 at the CCSD(T)/ANO level has been reported; see: Grev, R. S.; Schaefer, H. F., III. Organometallics 1992, 11, 3489.
the 6-31G* basis set (Table 1). In H2GedGeH2, digermene-to-germylgermylene isomerization is endothermic by 1.7 kcal mol-1 with a barrier of 9.5 kcal mol-1. For Me(H)GedGeH2, the isomerization barrier (19.8 kcal mol-1) is much higher than that of H2Ged GeH2, although the energy difference between Me(H)GedGeH2 and HG ¨ e-Ge(H2)Me (2.8 kcal mol-1 endothermic) is similar to that of H2GedGeH2 and HG ¨ eGeH3. This result suggests that alkyl groups are much more reluctant to migrate than hydrogen atoms. However, the isomerization of the parent 1,2-digermacyclohexa-1,4-diene to (germacyclopent-3-enyl)germylene is 3.2 kcal mol-1 exothermic, and the barrier is only 8.1 kcal mol-1, in sharp contrast to the Me group migration. Thus, this theoretical study shows that the ring contraction of 1,2-digermacyclohexa-1,4-diene is the favored reaction both kinetically and thermodynamically.13 Acknowledgment. This work was supported by Grants-in-Aid for Scientific Research (Nos. 12042213, 13029015, and 13440185) from the Ministry of Education, Science and Culture of Japan and the TARA (Tsukuba Advanced Research Alliance) fund. Supporting Information Available: Tables giving the details of the X-ray structure determination, and bond angles and figures giving thermal ellipsoid plots for 3b and 7 and figures and tables giving the calculated geometrical parameters of the transition state structure for the cyclic digermene and IRC results. This material is available free of charge via the Internet at http://pubs.acs.org. OM010369C (13) The migratory aptitude of silyl groups would be much larger than that of H and alkyl, as reported in disilene-silylene isomerization calculated by Nagase et al.; see: Nagase, S.; Kudo, T. Organometallics 1984, 3, 1320. However, the migration of the tBu3Si group in the real molecule was not observed due to steric reasons.
